So what does it take to be considered a good or even a great instructor?
Patience. Some folks get it right away, others don't. You must be able to teach them all.
Organization. You must be able to organize your class in a logical manner.
Experience. You need to have watched lots of divers with a critical eye. You need to know how things should be done as well as how they often are done poorly.
Confidence. You need to know you can handle things when they head south. You also need your students to believe in you.
Desire. You have to want to teach.
Open mind. Be willing to learn from everyone. You can learn from an idiot who is a terrible diver. You may be learning rescue techniques, but you'll still be learning.
Ability to say no. No, you didn't quite nail that skill, here's what you did wrong and here's how to fix it. No, you didn't buy a card, you bought instruction.
Willingness to go the extra mile. Be interested in your students. They may need extra help, give it to them.
Knowledge. You can't teach it if you never learned it.
Ability to teach. Most instructors don't understand how to teach.
"inadequacies in instruction that generally fall back on the instructor rather than the certifying agency."
If your agency leaves out skills and/or knowledge you can't teach it because you never learned it. Some go the extra mile and learn additional material and skills which they then teach to their students - most do not. The agency is a factor. An exceptional person can overcome a background based in an agency with low standards, but these folks are indeed rare. Most instructors are a product of their agency. Some agencies produce them with a cookie cutter.